Comparative Planetology: Gas Giants

Although the four gas‐giant planets are basically balls of hydrogen and helium gas and differ primarily only in mass, they have vastly different appearances. The progressive change of appearance in these planets, from the spectacular orange‐reddish banding and belting of Jupiter to the deep blue, nearly featureless appearance of Neptune, may be attributed to a single factor: their outer temperature. This temperature results from the balance between thermal radiation of the planet versus the absorption of solar energy. These outer planets also have differences in their overall makeup, due to differences in their net chemical composition and to the manner in which the various chemical elements can exist at the temperatures and pressures found in the planetary interiors (see Figure 1).

The approximately 60 moons in our solar system are found primarily in orbit about the gas‐giant planets. Because of the proximity of objects to each other and the relatively short time scales for gravitational modification of orbits, the lunar systems show many simple numerical relationships between their orbital periods (what astronomers term resonances). Ignoring the smallest objects, which appear to be debris from the collisional breakup of asteroids that has been captured into orbit after the formation of the planets, the moons are a distinct class of solar system object, chemically differentiated from both types of planets as well as other classes of objects in the solar system.

The four large moons of Jupiter, the so‐called Galilean moons Io, Europa, Callisto, and Ganymede, probably formed in association with the formation of Jupiter itself; but the remaining 12 smaller satellites are probably captured asteroids. These four major moons are in almost perfect gravitational resonance with each other. Over the history of the solar system, their mutual gravitational pulls have produced respective orbital periods of 1.769 days, 3.551 days, 7.155 days, and 16.69 days, with period ratios of 1.00:2.00:2.02:2.33.

The innermost two moons are rocky objects like Earth's Moon, though Europa appears to have an icy crust, which could overlie a deeper liquid ocean. The lower densities of the outer two moons (about 2.0 g/cm 3) suggest a composition of approximately half heavy elements (iron and silicates) and half ices (solid water, carbon dioxide, methane, and ammonia), which is typical of most of the moons about the gas giants. For a small object, Io is exceptional. Only slightly larger than Earth's Moon, it would be expected to have cooled and frozen long ago, but it is actually the most volcanic object in the solar system. The source of energy that keeps its interior molten is the changing gravitational tides produced by Europa as Io sweeps by on its inner orbit every three and a half days. The gases released from volcanoes on Io have produced a donut‐like belt of tenuous sulfur and sodium atoms about Jupiter. There also is evidence of ancient surface activity on Ganymede, which suggests that it too may have experienced some tidal heatings. Callisto, on the other hand, may have solidified so quickly that its heavier elements could not sink into the interior to form a core denser than the mantle.

Saturn has the largest family of moons whose compositions are again various combinations of rocky material and ice and whose orbits show many resonance relationships. These relationships include period‐period resonances between moons in different orbits and also 1:1 resonances, where a smaller object may be trapped 60 degrees ahead or behind in the orbit of a larger object. For example, the small moons Telesto (25 km diameter) and Calypso (25 km) are trapped by Tethys (1048 km) in its orbits. Janus and Epimetheus share nearly the same orbit, switching places every time the inner one catches up to the outer one.

Saturn's large moon, Titan, has the densest atmosphere (mostly nitrogen with some methane and hydrogen) of any satellite. With a surface pressure about 40 percent that of Earth, this produces a greenhouse effect temperature of 150 K — about twice the expected value based only on absorption of sunlight.

Orbiting Uranus are four largish (radii 580–760 km) and one intermediate size (radius 235 km) moons, with about ten known smaller objects. This lunar family includes Miranda, probably the most bizarre object among all solar system satellites. Its surface shows evidence of past cataclysmic events (was it broken up in a collision and reassembled?), and possibly it is in the process of readjusting to an equilibrium structure as lighter ices rise and heavier materials sink. Contrary to expectation, the planet's moons do not show resonances between their orbital periods.

Neptune's lunar system is unusual in that its largest moon, Triton, is in a retrograde orbit tilted 23 degrees with respect to the planet's equator, and a second moon, Nereid, is in a very elongated orbit. Tidal stresses imposed on Triton by Neptune have caused internal heating and alteration of its icy surface, eliminating ancient craters. Its surface appears unique in that activity there is in the form of geysers — at a surface temperature of 37 K, absorption of sunlight vaporizes frozen nitrogen below the surface, which escapes by forcing itself through the overlying ices. Because the Moon orbits in a direction opposite to the rotation of the planet, tidal effects also are decelerating its motion, causing it to slowly spiral in toward the planet. Triton will move within Neptune's Roche Limit in perhaps 100 million years and be destroyed, and its material will be dispersed in a Saturn‐like ring system. This suggests that Triton possibly was captured relatively recently, originally into an elliptical orbit that has been circularized by tidal effects.

Rings

All four of the outer planets in our solar system have rings composed of particles as small as dust to boulder‐size materials orbiting in their equatorial planes. Jupiter is encircled by a tenuous ring of silicate dust, probably originating from particles chipped off the inner moons by the impact of micrometeorites. Uranus is orbited by 11 optically invisible, thin rings composed of boulder‐size, dark particles; and Neptune has three thin and two broad rings, also composed of dark particles. The particles in the thin rings are unable to disperse due to the presence of shepherd moons, pairs of small moons only a few kilometers in diameter orbiting near the inner and outer edges of the rings. The shepherd moons' gravitational action confines small particles into a narrow ring at an intermediate orbital radius. The ring particles of Uranus and Neptune are dark because they are covered with dark organic compounds produced by chemical reactions involving methane.

It is Saturn that possesses the most extensive and obvious ring system, some 274,000 kilometers in diameter (see Figure 2). As seen from Earth, there is an apparent inner ring that extends inward to the top of the planet's atmosphere. Exterior to a large gap is a faint (or crape) ring, then a middle bright ring with a thin gap, the prominent Cassini's Gap, and finally an outer ring, Enke's Gap. Both the pattern of circular velocities as well as Earth‐based radar studies show that the rings are composed of myriads of small particles, each orbiting as a tiny moon. These are highly reflective icy particles, from a few centimeters in size to a few meters in size.

Figure 2

Saturn's ring system.

The rings of all the outer planets lie within each planet's Roche limit, the radial distance interior to which materials cannot coalesce into a single object under their own gravitation. In other words, the contrary gravitational pull on particles by the opposite sides of the planet is greater than the self‐gravity between particles. If a satellite were to pass closer to the planet than the Roche limit (about 2.4 planetary diameters, depending upon the size, density, and structural strength of the satellite), it would be broken apart by the gravitational forces of the planet (another example of which are tidal forces).

The ring system of Saturn further illustrates the great variety of dynamical phenomena that are the result of gravitational attraction between systems of particles of greatly differing masses. First, the planet has an equatorial bulge; the slight excess of mass about the equator gravitationally perturbs the orbits of smaller objects (from dust particles to moons) into its equatorial plane; hence the ring system is flat. Most of the gaps in the rings (small particles) are due to orbital resonances with the larger satellites. For example, the moon Mimas produces Cassini's Gap where particles otherwise would be orbiting the planet with half that moon's orbital period. Enke's Gap, however, is the result of a clearing of particles by a small moon that orbits at that distance from the planet. That Saturn's ring system is composed of thousands of such rings also suggests that there are numerous shepherd moons, only a few of which have been discovered.